Particle filter for partially enclosed microelectromechanical systems

ABSTRACT

A particle filter for a partially enclosed microelectromechanical systems that include a substrate material having at least one micro-device formed thereon. The particle filter includes a first structural layer forming a filter bottom and a second structural layer forming a filter wall. The filter bottom and filter wall are interconnected by at least one support feature to define a particle trap between the filter wall and filter bottom. The particle trap is a gap formed by mating, but non-interconnected portions of the filter wall and filter bottom that operates to trap and prevent particles from passing beyond the filter bottom into the microelectromechanical system.

RELATED APPLICATIONS

[0001] This patent application claims priority from U.S. patentapplication Ser. No. 10/224,179 that was filed on Aug. 20, 2002,entitled “PARTICLE FILTER FOR PARTIALLY ENCLOSED MICROELECTROMECHANICALSYSTEMS”. The entire disclosure of U.S. patent application Ser. No.10/224,179 is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention is related to the field of microelectromechanicalsystems, and, in particular, to a particle filter for a partiallyenclosed microelectromechanical system that reduces or preventsparticulate contamination of the micro-devices that make up the system.

BACKGROUND OF THE INVENTION

[0003] There are a number of fabrication technologies, collectivelyknown as micromachining, for producing microelectromechanical systems.One type of micromachining process is surface micromachining. Surfacemicromachining involves deposition and photolithographic patterning ofalternate layers of structural material (typically polycrystallinesilicone, termed polysilicon) and sacrificial layers (typically silicondioxide, termed oxide) on a silicon wafer substrate material. Using aseries of deposition and patterning, functional devices are constructedlayer by layer. After a device is completed, it is released by removingall or some of the remaining sacrificial material by exposure to aselective etchant such as hydrofluoric acid, which does notsubstantially attack the polysilicon layers.

[0004] Unfortunately, it is a problem in the art ofmicroelectromechanical systems to prevent particle contamination.Particle contamination can potentially ruin an entire system byinterfering with the electrical signals and/or mechanical movements ofsome or all of the electrical and/or mechanical devices.

[0005] One solution to this problem is to provide a cover over themicroelectromechanical system that at least partially encloses thesystem and protects enclosed components from particle contamination.When covers are utilized or otherwise when there are structural featureshaving substantial coverage area, etch release apertures in suchstructure are typically utilized to introduce etchant for removal of thesacrificial material and release of internal devices. These etch releaseapertures typically include openings on the order of about 1.25 micronsin size. Unfortunately, however, these openings still permit theintroduction of particles that are large enough to cause mechanicalobstructions or electrical shorts in the internal devices.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the present invention to provide aparticle filter and method of fabricating the same formicroelectromechanical systems that are at least partially enclosed by acover or other similar structure. It is another object of the presentinvention to provide a plurality of configurations for the particlefilter to accommodate different spatial limitations withinmicroelectromechanical systems. It is still yet another object of thepresent invention to provide a particle filter that may be formed aroundetch release apertures in a cover to trap particles introduced throughsuch apertures within the filter, thereby preventing contamination ofinternal components.

[0007] In carrying out the above objects, and other objects, features,and advantages of the present invention, a particle filter is providedthat includes a first structural layer forming a filter bottom and asecond structural layer forming a filter wall. The filter bottom andfilter wall are interconnected by at least one support feature to definea particle trap between the filter wall and filter bottom. In thatregard, the particle trap may be a gap formed by mating, butnon-interconnected portions of the filter wall and filter bottom. Theparticle trap operates to trap particles within the gap to preventparticles from passing beyond the filter bottom and into themicroelectromechanical system.

[0008] Various refinements exist of the features noted in relation tothe subject particle filter. Further features may also be incorporatedinto the particle filter to form multiple examples of the presentinvention. These refinements and additional features will be apparentfrom the following description and may exist individually or in anycombination. For instance, the particle filter may also include a filtertop. In this regard, the filter wall may be formed as part of the filtertop, which in turn is a portion of a cover for themicroelectromechanical system. Further, in this regard, the particlefilter may be formed so that the filter wall encloses an areacircumscribing one or more etch release apertures formed in the cover toprevent particulate contamination through the same.

[0009] The filter bottom on the other hand, may be formed in a pluralityof geometric configurations to accommodate spatial limitations within amicroelectromechanical system. In this regard, the filter wall mayoverlap a top portion of the filter bottom to define a particle trapthat includes a substantially right angle at the overlap of the filterwall and filter bottom to improve efficiency.

[0010] In carrying out the above objects, and other objects, features,and advantages of the present invention, a microelectromechanical systemis provided that includes at least a substrate material having at leastone micro-device formed on the substrate material. Themicroelectromechanical system also includes at least one particle filterto prevent particles from entering the microelectromechanical system.Various refinements exist of the features noted in relation to thesubject microelectromechanical system. Further features may also beincorporated into the microelectromechanical system to form multipleexamples of the present invention. These refinements and additionalfeatures will be apparent from the following description and may existindividually or in any combination. For instance, themicroelectromechanical system may also include a cover having at leastone etch release aperture. In this regard, the particle filter may beformed between the cover and the substrate material around the at leastone etch release aperture to prevent particulate contamination throughthe etch release aperture.

[0011] In carrying out the above objects, and other objects, features,and advantages of the present invention, a method of fabricating aparticle filter for a microelectromechanical system is provided. Themethod includes the step of depositing and patterning a plurality ofalternating layers of filter forming material and sacrificial materialon a substrate material to form at least one filter bottom and at leastone filter wall. The method also includes removing the sacrificialmaterial to release the at least one filter bottom and the at least onefilter wall to define a particle trap between mating butnon-interconnected portions of the filter bottom and the filter wall.

[0012] Various refinements exist of the features noted in relation tothe present method. Further features may also be incorporated into thepresent method to form multiple examples of the invention. Theserefinements and additional features will be apparent from the followingdescription and may exist individually or in any combination. Forinstance, the filter bottom and filter wall may be interconnected by atleast one support feature. In another instance, the method may furtherinclude forming the filter wall as part of the filter top, which in turnis a portion of a cover for the microelectromechanical system. Further,in this regard, the particle filter may be formed so that the filterwall encloses an area circumscribing one or more etch release aperturesformed in the cover to prevent particulate contamination through thesame. The method may further include patterning the filter bottom into apredetermined one of a plurality of geometric configurations toaccommodate spatial limitations in a microelectromechanical system. Inthis regard, the filter wall may overlap a top portion of the filterbottom to define a particle trap that includes a substantially rightangle at the overlap of the filter wall and filter bottom to improveefficiency.

[0013] In the context of the present invention, the first, second, andthird, etc. connotations used in reference to the layers are used forthe purpose of differentiating between different layers and are not usedto indicate a fabrication sequence or structural sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 illustrates an example of a microelectromechanical systemconfigured with a filter system according to the present invention;

[0015]FIG. 2 illustrates an example of a filter system according to thepresent invention;

[0016]FIG. 3 illustrates an example of the fabrication of the filtersystem of FIG. 2;

[0017]FIG. 4 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0018]FIG. 5 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0019]FIG. 6 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0020]FIG. 7 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0021]FIG. 8 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0022]FIG. 9 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0023]FIG. 10 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0024]FIG. 11 illustrates additional details of the fabrication of thefilter system of FIG. 2;

[0025]FIG. 12 illustrates another example of a filter system accordingto the present invention;

[0026]FIG. 13 illustrates another example of a filter system accordingto the present invention;

[0027]FIG. 14 illustrates another example of a filter system accordingto the present invention; and

[0028]FIG. 15 illustrates another example of a filter system accordingto the present invention.

DETAILED DESCRIPTION

[0029] Reference will now be made to the accompanying drawings, which atleast assist in illustrating the various pertinent features of thepresent invention. For purposes of illustration, the followingdescription is related to the formation of particle filters formicroelectromechanical (MEM) systems, although it will be appreciatedthat the present particle filters are easily formed and useful for bothmicromechanical and microelectromechanical systems. In addition, one ormore micro-devices or microstructures may define any givenmicromechanical or microelectromechanical system.

[0030] Surface micromachining is a preferred type of technique forfabricating the particle filters described herein, although othertechniques may be utilized as well. Moreover, in certain instances itmay be desirable to use a combination of two or more fabricationtechniques to define a given MEM system. Since surface micromachining isthe preferred fabrication technique for the MEM systems describedherein, the basic principles of surface micromachining will first bedescribed. Initially, various surface micromachined microstructures andsurface micromachining techniques are disclosed in U.S. Pat. No.5,783,340, issued Jul. 21, 1998, and entitled “METHOD FORPHOTOLITHOGRAPHIC DEFINITION OF RECESSED FEATURES ON A SEMICONDUCTORWAFER UTILIZING AUTO-FOCUSING ALIGNMENT”; U.S. Pat. No. 5,798,283,issued Aug. 25, 1998, and entitled “METHOD FOR INTEGRATINGMICROELECTROMECHANICAL DEVICES WITH ELECTRONIC CIRCUITRY; U.S. Pat. No.5,804,084, issued Sep. 8, 1998, and entitled “USE OF CHEMICAL MECHANICALPOLISHING IN MICROMACHINING”; U.S. Pat. No. 5,867,302, issued Feb.2,1999, and entitled “BISTABLE MICROELECTROMECHANICAL ACTUATOR”; andU.S. Pat. No. 6,082,208, issued Jul. 4, 2000, and entitled “METHOD FORFABRICATING FIVE-LEVEL MICROELECTROMECHANICAL STRUCTURES ANDMICROELECTROMECHANICAL TRANSMISSION FORMED, the entire disclosures ofwhich are incorporated by reference in their entirety herein.

[0031] Surface micromachining generally entails depositing typicallyalternate layers of structural material and sacrificial material usingan appropriate substrate which functions as the foundation for theresulting microstructures. A dielectric isolation layer will typicallybe formed directly on an upper surface of the substrate on which a MEMsystem is to be fabricated, and a structural layer will be formeddirectly on an upper surface of the dielectric isolation layer. Thisparticular structural layer is typically patterned and utilized forestablishing various electrical interconnections for the MEM system,which is thereafter fabricated thereon. Other layers of sacrificial andstructural materials are then sequentially deposited to define thevarious microstructures and devices of the MEM system. Variouspatterning operations may be executed on one or more of these layersbefore the next layer is deposited to define the desired microstructure.After the various microstructures are defined in this general manner,the desired portions of the various sacrificial layers are removed byexposing the “stack” to one or more etchants. This is commonly called“releasing.” During releasing, at least certain of the microstructuresare released from the substrate to allow some degree of relativemovement between the microstructure(s) and the substrate. In certainsituations, not all of the sacrificial material used in the fabricationis removed during the release. For instance, sacrificial material may beencased within a structural material to define a microstructure withdesired characteristics (e.g., a prestressed elevator microstructure).

[0032] Surface micromachining can be done with an y suitable system of asubstrate, sacrificial film(s) or layer(s), and structural film(s) orlayer(s). Many substrate materials may be used in surface micromachiningoperations, although the tendency is to use silicon wafers because oftheir ubiquitous presence and availability. The substrate again isessentially a foundation on which the microstructures are fabricated.This foundation material must be stable to the processes that are beingused to define the microstructure(s) and cannot adversely affect theprocessing of the sacrificial/structural films that are being used todefine the microstructure(s). With regard to the sacrificial andstructural films, the primary differentiating factor is a selectivitydifference between the sacrificial and structural films to thedesired/required release etchant(s). This selectivity ratio may be fiveto one or even less but is preferably several hundred to one or muchgreater, with an infinite selectivity ratio being ideal. Examples ofsuch a sacrificial film/structural film system include: various siliconoxides/various forms of silicon; poly germanium/poly germanium-silicon;various polymeric films/various metal films (e.g.,photoresist/aluminum); various metals/various metals (e.g.,aluminum/nickel); polysilicon/silicon carbide; siliconedioxide/polysilicon (i.e., using a different release etchant likepotassium hydroxide, for example).

[0033]FIG. 1 illustrates an exemplary MEM system 100 configured withmultiple particle filters, e.g. 102, 104, and 106 according to thepresent invention. MEM systems constructed by MEMX, Inc. of Albuquerque,N. Mex., such as MEM system 100 may include a first layer 108 thatprovides electrical interconnections and as many as five or moreadditional layers of mechanical polysilicon layers that form functionalelements ranging from simple cantilevered beams to complex microenginesconnected to a gear train. MEM system 100 also includes a cover 110 toprotect the electrical and mechanical layers 108 and 112-116 fromparticle contamination. Etch release apertures 118A-F in the cover 110provide a means to introduce etchant during the release step to removethe remaining sacrificial material and release the mechanical andelectrical devices in the layers 108 and 112-116. Such etch releaseapertures facilitate penetration of the etchant for improved yield. Theetch release apertures 118A-F are typically on the order of about 1.25microns in size. The particle filters, e.g. 102-106, are preferablyformed around the etch release apertures 118A-F and operate to trapparticles that may enter the MEM system 100 through the apertures118A-F, thereby assuring that virtually no contamination may occur inthe MEM system 100. The filters, e.g., 102-106, thus allow penetrationof the etchant but impede ingress of particles of a size that mayobstruct movement or cause short circuits.

[0034]FIG. 2 illustrates a cut away perspective view of the particlefilter 102. For purpose of illustration, the following description willnow be directed toward the operation and fabrication of the illustratedparticle filter 102, having an exemplary configuration and associatedfabrication process. It will be appreciated however, that the followingdiscussion applies equally to the particle filters 104 and 106, as wellas other particle filters described herein, as well as otherconfigurations and processes according to the invention.

[0035] The particle filter 102 includes a filter bottom 200 and filterwall 202. The filter wall 202 is interconnected to the filter bottom 200by support feature 206, referred to herein as anchor post 206. Thefilter wall 202 may also be formed from at least one depending portionof the cover 110 over MEM system 100. In other words, a filter top maybe provided by forming the filter wall 202 and cover 110 from the samedeposition layer or integrally or otherwise interconnected layerportions in the MEM system 100.

[0036] In that regard, the filter wall 202 and filter bottom 200 definea particle trap 208 formed at the mating but non-sealably interconnectedintersection of the filter wall 202 and filter bottom 200. That is, thefilter wall 202 and bottom 200 interface so as to provide one or moreopenings dimensioned to allow penetration of etchant but capture certainparticles that may have passed through an etchant aperture, e.g., 118A.As illustrated on FIG. 2, the filter wall 202 and filter bottom 200 arenot actually connected, but rather, define a gap or space along theintersection that forms the particle trap 208. In this case, the anchorpost 206 provides the interconnection between the filter wall 202 andfilter bottom 200, via the filter top/cover 110. As may be appreciated,the dimension of the gap 208 is defined by the size of particle to betrapped within the filter 102. In this regard, the dimension of the gap208 is preferably, in the range of 0.1 microns to 0.5 microns, and morepreferably is 0.2 microns. Operationally, the particle trap 208effectively traps particles entering the particle filter 102 within thegap 208, thereby preventing the particles from contaminating themechanical and electrical micro-devices in the layers 108 and 112-116.

[0037]FIGS. 3-11 Illustrate one example of the fabrication of theparticle filter 102. Only those portions of the MEM system 100 that arerelevant to the present invention will be described herein. Thoseskilled in the art will appreciate, however, that since the particlefilter 102 is preferably fabricated using micromachining, various othercombinations of depositions and surface machining that are within thescope of the present invention exist to produce particle filtersaccording to the principles disclosed herein.

[0038] Referring first to FIG. 3, there is shown a cross sectional viewof the fabrication process for the particle filter 102 completed to thestructural layer 310 forming the filter bottom 200. Specifically, thestructure of FIG. 3 includes a substrate 300, dielectric isolationlayers, 302 and 304, a pair of sacrificial layers, 306 and 308, and astructural layer 310. It should be noted that in the context ofproducing the MEM system 100 the sacrificial layers, 306 and 308, may bestructural layers such as structural layers 114 and 116. However, forpurposes of clarity, the fabrication of the particle filter 102 isillustrated in FIGS. 3-11 utilizing sacrificial layers 306 and 308. Inother words, to provide a clearer understanding of the presentinvention, sacrificial layers, 306 and 308, are shown on FIGS. 3-11rather than structural layers 114 and 116.

[0039] The dielectric isolation layers, 302 and 304, may be a thermaloxide layer and silicon nitride layer respectively, formed by aconventional thermal diffusion process as is well known in theintegrated circuit art. In addition, chemical-mechanical polishing maybe utilized to adjust the thickness and planarity of the layers, e.g.layers 302-310. The term “substrate” as used herein means those types ofstructures that can be handled by the types of equipment and processesthat are used to fabricate microdevices and/or microstructures on,within, and/or from a substrate using one or moremicro-photolithographic patterns.

[0040] Exemplary materials for the sacrificial layers, 306 and 308, aswell as other sacrificial layers utilized to form the particle filter102 include undoped silicon dioxide or silicon oxide, and doped silicondioxide or silicon oxide (“doped” indicating that additional elementalmaterials are added to the film during or after deposition). Exemplarymaterials for the structural layer 310 as well as other structurallayers that form the particle filter 102 include doped or undopedpolysilicon and doped or undoped silicon. Exemplary materials for thesubstrate 300 include silicon. The various layers described herein maybe formed/deposited by techniques such as chemical vapor deposition(CVD) and including low-pressure CVD (LPCVD), atmospheric-pressure CVD(APCVD), and plasma-enhanced CVD (PECVD), thermal oxidation processes,and physical vapor deposition (PVD), and including evaporative PVD, andsputtering PVD, and chemical-mechanical polishing (CMP) as examples.

[0041] After formation of the structure of FIG. 3, the structural layer310 may be patterned using photolithographic masking and etching intothe shape of the filter bottom 200, as illustrated in FIG. 4. In thisregard and, a thin layer of light sensitive photoresist may be spun ontothe layer 310. The layer 310 may then be exposed to light using an oxidemask. After etching, the remaining photoresist may then be stripped awayresulting in the structure of FIG. 4. As will become apparent from thefollowing description, the filter bottom 200 may be patterned into avariety of shapes as a matter of design choice to accommodate differentspatial configurations and limitations within a MEM system, such as MEMsystem 100.

[0042] Referring to FIG. 5, after patterning of the filter bottom 200,another layer 500 of sacrificial material is deposited onto thepatterned layer 310. It should be noted, however, that while thesacrificial layer 500 is shown in a planarized state, such as could beachieved through chemical-mechanical polishing, planarization is notnecessary to the fabrication of the particle filter 102. Referring toFIG. 6, the sacrificial layer 500 is patterned using a cut etch to forma circumferential annular void 600 within the sacrificial layer 500. Thecircumferential annular void 600 will eventually become the filter wall204 for the particle filter 102. It should also be noted that the void600 is etched all the way down to the structural layer 310/filter bottom200 and slightly overlaps the side of the structural layer 310 or inother words the top portion of the filter bottom 200. The overlap is notnecessary to the formation of the particle filter 102, but rather,increases the efficiency of the particle filter 102 as it forms the lip210 (shown on FIG. 2) of the particle trap 208, which further restrictsparticles passing through the particle trap 208.

[0043] Referring to FIG. 7, after etching of the void 600, a thin layerof sacrificial material 700 is applied to backfill void 600. Thethickness of the backfill layer 700 determines the gap spacing of theparticle trap 208 and therefore is precisely controlled during thebackfill process. In that regard, the thickness of the backfill layer700 is preferably in the range of 0.1 microns to 0.5 microns and morepreferably is 0.2 microns. It should also be noted since the layer 700is generally the same or similar material as the sacrificial layer 500it essentially becomes part of the layer 500 as shown in FIG. 8.Alternatively a timed etch to the desired depth may be utilized to formthe void 600, thus eliminating the need for the backfill layer 700. Aswill be appreciated by those skilled in the art, however, the backfillmethod eliminates many of the difficulties associated with timedetching, e.g. knowledge of the precise thickness of the sacrificiallayer 500. Still referring to FIG. 8, the sacrificial layer 500including the added material of layer 700 is again patterned using a cutetch to form a substantially central annular void 800. The centralannular void 800 will eventually become the anchor post 206 for theparticle filter 102.

[0044] Referring to FIG. 9, after the sacrificial backfill layer 700 isdeposited and void 800 etched, another structural layer 900 is depositedand planarized. Again as will be appreciated the planarization is notnecessary to the formation and/or operation of the particle filter 102.The structural layer 900 forms the filter wall 202, anchor 206, and thetop cover 110. Referring to FIG. 10, after deposition and planarizationof the layer 900, etch release apertures 118A are cut into the layer 900to provide the means for introducing the chemical etchant used torelease the particle filter 102 and or other microdevices and/ormicrostructures in a MEM system, such as MEM system 100.

[0045] Referring to FIG. 11, the etch release step utilizes a selectiveetchant that etches away exposed portions of the sacrificial layers 306,308, and 500 over time, while leaving the polysilicon structural layers302, 304, and 310 intact to form/release the particle filter 102.Examples of release etchants for silicon dioxide and silicon oxidesacrificial materials are typically hydrofluoric (HF) acid based (e.g.,undiluted or concentrated HF acid, which is actually 49 wt % HF acid and51 wt % water; concentrated HF acid with water; buffered HF acid (HFacid and ammonium fluoride)).

[0046] The completed particle filter 102 is supported in the MEM system100 by the filter top/cover 110, which in turn supports the filterbottom 200 via the anchor post 206. Advantageously, this permits theformation of the particle trap 208 around the etch release apertures118A. Also advantageously, in this regard, the particle filter 102virtually eliminates the possibility of particle contamination asparticles entering through the etch release apertures 118A are trappedby the particle trap 208. As stated above, the etch release aperturesare on the order 1.25 microns in size while the particle trap is on theorder of 0.2 microns in size.

[0047] Referring to FIGS. 12-15, a further advantage of the presentinvention is provided through various alternative embodiments of thepresent particle filter. The present particle filter can be constructedin a variety of geometrical shapes as a matter of design choice. Thoseskilled in the art will appreciate the slight variations in etching toachieve the various designs illustrated in FIGS. 12-15, and thus, adescription is omitted for the purpose of brevity. Additionally, thoseskilled in the art will appreciate that the particle filters 1200-1500are for purpose of illustration and not limitation and that numerousother designs can be formed according to the principles of the presentinvention.

[0048] The particle filters 1200-1500 operate substantially similarly tothe particle filter 102 in that they include a particle trap defined bymating, but non-interconnected surfaces, of a filter wall and a filterbottom connected to the filter wall through a support feature. Theparticle filters 1200-1500, however, provide the advantage ofaccommodating various different spatial limitations created by thedifferent microstructures that can be included in a MEM system such asMEM system 100. For example, particle filter 1300 includes a slightlysmaller filter bottom 1302 and is externally supported by an anchor post1304. Particle filters 1200, 1400 and 1500 all include variations of theprinciples of the present invention and may be incorporated into one ormore MEM systems as a matter of design choice. In addition, it will beappreciated that a MEM system, such as system 100, could include one ormore of the different filter designs, e.g. 102, and 1200-1500, in asingle system as a matter of design choice.

[0049] Those skilled in the art will appreciate variations of theabove-described embodiments that fall within the scope of the invention.As a result, the invention is not limited to the specific examples andillustrations discussed above, but only by the following claims andtheir equivalents.

We claim:
 1. A method for constructing a particle filter in amicroelectromechanical system, the method comprising: depositing andpatterning a plurality of alternating layers of filter forming materialand sacrificial material on a substrate material to form at least onefilter bottom and at least one filter wall; and removing the sacrificialmaterial to release the at least one filter bottom and the at least onefilter wall to define a particle trap between interfacing portions ofthe filter bottom and filter wall.
 2. The method of claim 1 wherein theat least one filter bottom and filter wall are interconnected by atleast one support feature.
 3. The method of claim 1 wherein the step ofdepositing and patterning the plurality of alternating layers of filterforming material and sacrificial material comprises: forming a filtertop including at least one etch release aperture.
 4. The method of claim3 wherein the filter wall encloses an area circumscribing the at leastone etch release aperture.
 5. The method of claim 1 comprising:patterning the filter bottom into a predetermined one of a plurality ofgeometric configurations.
 6. The method of claim 1 wherein the particletrap comprises: a gap of pre-determined dimension formed between matingbut non-interconnected portions of the filter bottom and filter wall. 7.The method of claim 6 wherein the filter wall at least partiallycircumscribes a top portion of the filter bottom to define the particletrap.
 8. The method of claim 7 wherein the filter wall overlaps the topportion of the filter bottom to form the gap between mating butnon-interconnected portions of the filter bottom and filter wall.